The present invention relates generally to lithography systems for semiconductor fabrication, and more specifically to reducing carbon contamination in sub-atmospheric charged particle beam lithography systems.
Photolithography has become a critical enabling technology in the fabrication of modem integrated circuit (IC) devices. The photolithography process typically involves exposing a patterned mask to collimated radiation to produce patterned radiation. The patterned radiation is then passed through an optical reduction system, and the reduced patterned radiation or mask image is projected onto a substrate, which is typically a silicon wafer upon which other materials are deposited or incorporated. For fabrication using photolithography processes, the substrate is typically coated with photoresist. The radiation exposure changes the properties of the photoresist and allows subsequent processing of the substrate.
As the degree of circuit integration has increased, the feature sizes of IC""s have dramatically decreased. To support future semiconductor fabrication requirements, lithography systems using charged particle beams, such as electron beams or ion beams, have been developed to overcome limitations of traditional optical systems. In charged particle beam projection lithography systems, portions of a mask are illuminated with a charged particle beam to project an image of the mask onto a substrate. Several new charged particle beam lithography systems have been developed to extend lithography capabilities to sub-0.15 micron feature size levels. One such system is a microcolumn electron beam system developed by IBM. This system uses a large number of miniature electron beam writers in a phased array to project mask images on the order of 0.1 micron wafer geometries. Another such system is a SCALPEL(copyright) electron beam lithography system developed by ATandT Bell Laboratories; SCALPEL, stands for xe2x80x9cScattering with Angular Limitation in Projection Electron-beam Lithographyxe2x80x9d and is a registered trademark of ATandT Bell Laboratories of Murray Hill, N.J. The SCALPEL lithography system projects high-energy electrons through an electron beam scattering mask. A similar lithography system for next generation sub-0.1 micron circuits, is the PREVAIL (Projection-Reduction Exposure with Variable Immersion Layer) system recently developed by IBM.
A significant disadvantage associated with present charged particle beam lithography systems is a susceptibility to carbon, or carbon-containing material, contamination on the mask, walls, and other surfaces of the imaging (or process) chamber of the lithography system. Repeated exposure of a mask to charged particle beams tends to cause a build-up of carbon on the mask if sufficiently high enough levels of carbon contaminants are present in the imaging chamber. FIG. 1A illustrates a side view of a mask, such as a mask used in a SCALPEL lithography system. The mask has an image pattern formed on it and is used to project a reduced image pattern onto a substrate. The critical dimension (CD) represents the smallest feature size or spacing between features routinely produced on the substrate forming the integrated circuit device. The CD is controlled by feature 102 on mask 100 which has a dimension 104, and which produces the CD on the wafer. For a mask used in the SCALPEL lithography system, feature 102 represents a xe2x80x9cblockingxe2x80x9d region and surface 106 represents a transparent region. A blocking region scatters incident electrons and a transparent region transmits incident electrons.
Carbon deposits on the mask or on the wafer may cause the CD to change (usually to increase). This effect is illustrated in FIG. 1B, in which carbon deposit 112 on feature 102 has increased the dimension of feature 102 from a first width 104 to a second width 114. Carbon deposits on the features of the mask can also cause heating of the mask features due to excessive absorption, rather than scattering, of incident electrons. Such heating can cause the mask features to become undesirably distorted.
Even in cases in which the degree of carbon deposition on the mask is not significant enough to alter the CD, the presence of contaminants on the mask and aperture surfaces can cause unwanted deflection of an electron beam that passes through the mask and/or other apertures. With reference to FIG. 1B, carbon deposit 116 on the transparent region of the mask 100 can cause incident electrons to be undesirably scattered and thereby produce an unwanted image on the wafer. Thus, even minute amounts of carbon contamination can adversely affect the imaging process within electron beam lithography systems.
Newer semiconductor lithography tools may use micro-column array (MCA) electron beam systems for wafer fabrication, inspection, and/or metrology. These systems are built by arranging a number of single micro-column electron sources in either linear or rectangular arrays. MCA lithography systems typically image a stored pattern directly onto a resist coated mask substrate (e.g., Cr on glass), as opposed to imaging through a projection mask. Carbon contamination can deposit onto the resist surface and absorb some of the incident energy, thereby affecting the exposure dose. In MCA systems, carbon contamination can also clog the apertures in the imaging system. Such aperture clogging and dose alteration can cause serious problems for present MCA lithography systems.
Carbon contamination can also pose a problem in charged particle beam (typically, electron beam) metrology systems. Carbon deposits in metrology systems usually result in line widening which results in the measured CD being larger than the true CD due to the built up layer of carbon. The same type of problem also effects various defect review tools that are used in the inspection and production of semiconductor wafers.
Carbon contamination can be introduced by various sources into the imaging chamber that contains the electron source, mask, wafer, and imaging components of a charged particle beam lithography system. These sources of contamination include hydrocarbon vapor that might be present in the chambers or carbon residue in the wiring or electron sources, stage supports, and other similar sources. While it is possible to reduce carbon contamination by ensuring that exclusively non-carbon materials are used in the imaging chamber, or by physically cleaning the chamber and mask often and thoroughly, such measures are typically very expensive and time consuming, and limit the throughput of the lithography system. Moreover, such methods may not totally remove the carbon contamination in the imaging chamber, since organic compounds are often present in the masks and wafers themselves.
In light of the above, there is a need in the art for a system that removes carbon contamination deposited within the chambers of a charged particle beam photolithography apparatus.
The present invention relates to a cleaning system that prevents build-up of carbon deposits in an imaging chamber of a sub-atmospheric charged particle beam lithography system. In particular, the invention relates to a cleaning system that eliminates built-up carbon deposits on mask and imaging chamber surfaces in the sub-atmospheric charged particle beam lithography system comprising an array of multiple charged particle beam sources.
A cleaning system for use with a sub-atmospheric charged particle beam lithography system is described. In one embodiment of the present invention, the cleaning system includes an oxidizer source that introduces an oxidizer into an imaging chamber of the lithography system to oxidize carbon contamination that has built up on the surfaces within the chamber, such as the mask and chamber walls. A volatile gas species comprising oxidized carbon gas is produced from the oxidization process. This volatile gas is pumped out of the chamber by a vacuum pump coupled to the imaging chamber, to thereby remove the carbon contaminant deposits from the imaging chamber.
In one embodiment of the present invention, a cleaning process comprises directing an oxidizer, such as oxygen gas, across the mask to remove carbon deposits from the mask while the mask is situated in the imaging chamber and the chamber is maintained at a sub-atmospheric pressure. In accordance with this embodiment of the present invention, the cleaning process is performed periodically after a predetermined number of wafers have been processed. In an alternative embodiment of the present invention, the cleaning process is performed continuously by introducing an oxidizer into the imaging chamber while each wafer is being processed.